1. Field of the Invention
The present invention relates to an optical element driving device which is used as an optical head in an optical disc device for performing recording or playback of information on information recording media (optical discs) such as a DVD (Digital Versatile Disc) recorder, and more particularly to a technique for constructing a system for holding a neutral position of an optical element.
2. Description of the Background Art
One of the known systems for holding a neutral position of an optical element is disclosed in Japanese Patent Application Laid-Open No. 7-110955. FIG. 13 is a plan view showing a prior-art objective lens driving device disclosed in this gazette. FIG. 14 is a longitudinal section showing the objective lens driving device of FIG. 13, and FIG. 15 is a perspective view showing a magnetic piece used for the objective lens driving device of FIG. 13.
In FIGS. 13 to 15, reference numeral 1 denotes a lens holder, 2 denotes an objective lens, 3 denotes a tracking driving coil, 4 denotes a focusing driving coil and 5 denotes an inner yoke. Further, reference numeral 6 denotes a magnet, reference sign 6a denotes a focusing magnet portion, 6b denotes a tracking magnet portion, 6c denotes a groove, reference numeral 7 denotes a fixed axle, 8 denotes a balancer, 9 denotes an outer yoke, reference sign 9a denotes a boss portion, reference numeral 10 denotes a magnetic piece, reference signs 10a , 10b denotes end portions and 10c denotes a center portion.
In FIGS. 13 to 15, the lens holder 1 is supported rotatably about the fixed axle 7 and movably along the fixed axle 7. The objective lens 2 and the balancer 8 are disposed at end portions of the lens holder 1, and the tracking driving coil 3 and the focusing driving coil 4 are fixed to an outer peripheral surface thereof. The fixed axle 7 is fixed to the boss portions 9a in the center portion of the outer yoke 9. The magnet 6 is fixed to an inner surface of the outer yoke 9 and consists of the focusing magnet portion 6a, the tracking magnet portion 6b and the groove 6c. The focusing magnet portion 6a is magnetically polarized so that the north pole and the south pole should be aligned in the direction of the fixed axle 7, and the tracking magnet portion 6b is magnetically polarized so that the north pole and the south pole should be aligned circumferentially. The inner yoke 5 is layered on the outer yoke 9.
Further, in the peripheral edge of the lens holder 1, the magnetic piece 10 is fixed to a position opposite to a magnetic core of the focusing magnet portion 6a. 
An operation of the present device will be discussed below. Specifically, by energizing the focusing driving coil 4, the objective lens 2 together with the lens holder 1 is moved in a focusing direction, i.e., in the axial direction of the fixed axle 7, for focusing control. Further, by energizing the tracking driving coil 3, the lens holder 1 is rotated about the fixed axle 7 to perform tracking control of the objective lens 2.
The prior-art objective lens driving device having the above constitution, however, has the following problems.
First, since two magnetic pieces are needed to exert the magnetic suction force for holding a neutral position of the objective lens, the cost of parts becomes higher and the number of fabrication steps increases.
Second, since two magnets for focusing control and two magnets for tracking control, total four magnets, are needed and it is necessary to magnetically polarize these magnets, the cost becomes higher.